The present invention relates generally to an exposure apparatus that exposes a single crystal substrate for a semiconductor wafer, a glass plate for a liquid crystal display (“LCD”), and more particularly to an exposure apparatus that uses the extreme ultraviolet (“EUV”) light for an exposure light source.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask (reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with recent demands for finer processing to the semiconductor devices, a shorter wavelength of ultraviolet (“UV”) light has been promoted from a ultra-pressure mercury lamp (i-line) (with a wavelength of approximately 365 nm), a KrF excimer laser (with a wavelength of approximately 248 nm) to an ArF excimer laser (with a wavelength of approximately 193 nm).
However, the lithography using the ultraviolet light has the limit to satisfy the rapidly promoting fine processing to the semiconductor device, and a reduction projection optical system using extreme ultraviolet (“EUV”) light with a wavelength of 5 to 20 nm shorter than that of the UV light (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
The light absorption in a material remarkably increases as the wavelength of the exposure light becomes shorter, and it is difficult to use a refractive element or a lens for the visible light and UV light. In addition, no glass material exists in a wave range of the EUV light, and a catoptric optical system uses only a reflective element, such as a multilayer coating mirror.
The mirror does not reflect all the exposure light, but absorbs the exposure light of 30% or greater. The absorbed exposure light heats and deforms the mirror, and deteriorates its optical performance, in particular, imaging performance. Therefore, the mirror is made of a low thermal expansion material with such a very small coefficient of thermal expansion (“CTE”) as 5 ppb/K, so as to reduce a mirror's deformation as the temperature changes.
The EUV exposure apparatus uses a one-tenth wavelength of the conventional UV light, and allows only one-tenth surface deformations of the conventional ones or about 0.1 nm rms or smaller. For example, when the mirror has a CTE of 5 ppb/K and a thickness of 100 mm, the mirror's surface deforms by 0.1 nm, reaching the permissible value of the deformation in the EUV exposure apparatus, as the temperature rises only by 0.2° C.